Mammalia: Classification 781 



specialized of land mammals. African fossils indicate the existence of 

 ancient li> racoids much larger and more numerous than modern conies. 

 Sirenia show definite ungulate characteristics. Such teeth as they have 

 resemble those of elephants, especially in the tendency to develop 

 incisor tusks. Fossils prove the existence of sirenians in northern 

 Africa in the Eocene. These ancient sea cows had a more nearly com- 

 plete dentition and had small, but probably functional, hindlimbs. 



Some classifications create a group, "Subungulata," to contain 

 Proboscidea and Hyracoidea. The rabbit-like conies and elephants 

 seem to be about as unlike as possible, but they possess at least a 

 common negative characteristic in that neither of them shows any 

 evidence of close relationship to either perissodactyl or artiodactyl 

 ungulates, and the African i'ossils give some positive basis for putting 

 the two groups together. The Sirenia also are sometimes included in 

 Subungulata. Sea cows probably have the same relation to terrestrial 

 ungulates that the seals and walrus have to terrestrial carnivores, but 

 the sea cows have gone so much farther in their aquatic adaptations 

 than have the pinnipeds that they seem to merit the status of a separate 

 Order — they have ceased to be literally "ungulate." 



The direct descent of Carnivora from the primitive Creodonta is 

 reasonably certain. Except in the teeth, modern Carnivora are not very 

 highly specialized, certainly far less so than most ungulates. Some 

 classifications include Creodonta as a suborder under Carnivora. The 

 Cetacea, existing and ancient, show no strong affinities to mammals 

 of any other order. The earliest whales known existed in the middle 

 Eocene in the northern African region. They were even then definitely 

 whales, but the skull lacked the extreme specialization of the modern 

 cetacean skull, resembling that of a creodont, and the teeth were of 

 creodont type in both their number and their form. The cetaceans, 

 still carnivorous in habit, possibly had origin from some early creodonts 

 which had already become carnivorous. 



Cetaceans, sirenians, and pinnipeds represent three independ- 

 ent parallel (?) lines of aquatic adaptation. Along the cetacean line, the 

 adaptations to the mechanical needs of highly efficient aquatic loco- 

 motion have been carried to their physiologic limit, at the expense of 

 complete loss of ability to go ashore, but giving these most highly 

 specialized of all mammals a capacity for speed, deep diving, and long 

 submergence which makes them masters of the "high seas" and cos- 

 mopolitan rovers over all the great oceans. The sirenians are relatively 

 clumsy swimmers and their herbivorous diet limits them to habitation 

 of coastal waters. The pinnipeds have departed least from their an- 

 cestral terrestrial anatomy. The> are fairly good swimmers but are 

 able to go ashore. (Fig. 599.) 



